1. Field of the Invention
This invention relates generally to automatic transmissions for automotive vehicles, in particular to gear ratio changes produced in such transmissions by control elements such as friction clutches and brakes.
2. Description of the Prior Art
It is difficult to calibrate excellent shift quality under all circumstances for coasting or power-off downshifts in synchronous automatic transmissions. Such calibrations are especially difficult under low-speed conditions where vehicle noise levels are low and gear ratio steps are largest. Coasting downshifts present the largest inhibitor to a fully synchronous automatic transmission design.
Control strategies require calibration flexibility to ensure consistent, smooth downshifts under all operating conditions, but conventional gear shift control strategies are insufficient to meet the current requirements for coasting shift quality and cost.
Incorrect clutch stroke pressure can result in shifts that tie-up or have excessive flare. Such shifts are very noticeable to the vehicle operator. Shifts that repeatedly tie-up or flare can also result in damage to the control elements.
Conventional stroke pressure adaptation methods rely on analyzing the behavior of the clutch controlling the ratio change of an automatic transmission. They adjust the stroke pressure based on the amount of time it takes to reach a predetermined percent of shift completion. Such methods work in most cases for the measured events, but they sometimes lead to incorrect adaptation when other events use the same information.
A need exists in the industry for a technique that evaluates a shift event, and adapts pressure applied to the non-controlling element, which previously had no evaluation for stroke pressure and could therefore not adapt out of a bad shift.